1. Field of the Invention
The present invention relates to a supersensitive magnetic detection device for detecting displacements and rotations etc. of an object to be detected, by making use of a change in resistance (resistance change) in a magnetoresistive effect element.
2. Discussion of the Background
Hitherto, a magnetic sensor has been widely utilized because of its compactness. Being provided with a bias magnet, the magnetic sensor operates to detect the displacements and rotations etc. of an object composed of magnetic material by making use of a resistance change in a magnetoresistive effect element.
For example, Japanese Unexamined Patent Publication (Kokai) No. 3-195970 discloses a prior art magnetic sensor of the above kind. FIG. 1 shows a principle for detection that the magnetic sensor disclosed in the publication No. 3-195970 adopts. The magnetic sensor of FIG. 1 is provided with a bias magnet 15 which produces a bias magnetic field 13 toward a gear 15 as an object to be detected, which is composed of magnetic material.
An insulating substrate 17a is disposed in a plane perpendicular to a direction of the bias magnetic field 13, and is provided with magnetoresistive effect elements 16a, 16b. Magnetic lines of force in the bias magnetic field generated from the bias magnet 15 are periodically modulated by peaks and troughs of the gear 11 and turned into a sinusoidal wave pattern, corresponding to relative positions of teeth of the gear 11.
A deflection angle .theta. of the bias magnetic field 13 changes with the movement of the gear 11. In the arrangement, the movement of the gear 11 is detected by detecting a change of magnetic field intensity in the direction of the deflection angle, which is caused by a change of the magnetic field angle and produced in a plane of the element 16b, as a resistance change in the element 16b.
FIG. 2 shows the second form of the magnetic sensor disclosed in the publication No. 3-195970. In FIG. 2, the magnetoresistive effect element 16b on an insulating substrate 17b is arranged in a plane containing a direction of the bias magnetic field 13 and a moving direction of the gear 11.
According to this form, by harnessing changes of an angle made by a magnetic direction due to the magnetic deflection and a current direction in the magnetoresistive effect element 16b, the magnetoresistive of the element 16b changes in response to state variations of the bias magnetic field 13 caused by the movement of the gear 11.
Thus, in either case of the above-mentioned first and second forms of the magnetic sensor, the direction of the bias magnetic field 13 is a direction directing the gear 11 and it makes use of a phenomenon where the direction of the magnetic field 13 deflects to the moving direction of the gear 11 in response to the movement of the gear 11.
In the conventional magnetic sensor, however, as an air gap between the gear 11 and the magnetoresistive effect elements 16a, 16b increases, the sensitivity of the magnetic sensor is lowered since the deflection angle decreases abruptly.
In order to prevent lowering the sensitivity of the sensor, the Applicant has already filled an application titled a magnetic detection device in Japanese Patent Application No. 8-172499, which is not published yet. The unpublished magnetic detection device is shown in FIG. 3.
In FIG. 3, both a first magnet 25a and a second magnet 25b produce bias magnetic fields against the gear 11. The second magnet 25b is arranged to oppose the first magnet 25a in a manner that each pole of the first magnet 25a faces a different kind of pole of the second magnet 25b. Further, the first and second magnets 25a, 25b are arranged along the moving direction of the gear 11.
The magnetoresistive effect element 16 is disposed in a magnetic field between the first magnet 25a and the second magnet 25b and arranged in a plane perpendicular to the moving direction of the gear 11, for producing the resistance change by state variations of the bias magnetic field in response to the movement of the gear 11.
In the magnetoresistive effect element 16, as shown in FIG. 4, the magnetoresistive effect element 16b is connected to the magnetoresistive effect element 16a in series. The element 16a is arranged in a direction perpendicular to a direction X directing the gear 11, the element 16b is arranged along the direction X.
According to the magnetic detection device constructed above, as shown in FIG. 5, the bias magnetic field in the moving direction of the gear 11 is produced in a space defined between the first magnet 25a and the second magnet 25b. Owing to the movement of the gear 11, the bias magnetic field is deflected and modulated by the deflection angle .theta. to the direction X facing the gear 11.
Therefore, the magnetoresistive effect element 16a arranged in the direction perpendicular to the direction X directing the gear 11 produces the resistance change due to the state variation of the bias magnetic field in response to the movement of the gear 11. Consequently, the reduction in magnetic modulation against the air gap between the gear 11 and the magnetoresistive element 16a is decreased relatively, whereby the sensitivity against the air gap can be improved.
Note, although it is preferable to miniaturize the magnetic detection device of FIG. 3, it is necessary to thin the magnets 25a, 25b in the moving direction of the gear 11 and reduce a distance between the magnets 25a and 25b for that purpose.
However, such a reduction of thickness of the magnets 25a, 25b makes it impossible to ensure a sufficient magnetic field intensity. Alternatively, when reducing the distance between the magnets 25a and 25b, the magnetic change is decreased. Consequently, the sensitivity of the sensor is lowered remarkably.
For example, in a case that the distance between two magnets each having 2 mm in thickness is 4 mm, as shown with marks .circle-solid. of FIG. 6, the sensitivity varies from about 64 mV to about 7 mV with respect to a detected gap of 0 to 3 mm. Further, in case that the distance between two magnets each having 1 mm in thickness is 2 mm, as shown with marks X of FIG. 6, the sensitivity varies from about 40 mV to about 5 mV with respect to a detected gap of 0.5 to 2 mm. Consequently, it is impossible to miniaturize the magnetic detection device shown in FIGS. 4 to 6.
In addition, an intensity of the magnetic field H greatly depends on a position of the direction X. Thus, under condition of arranging the magnetoresistive effect element 16 in the bias magnetic field, there is produced a difference in magnetic working point in the identical magnetoresistive effect element 16a, depending on the position. The magnetoresistive effect element 16a has a resistance changed by the magnetic filed intensity, as shown in FIG. 7.
Further, since the magnetic field varies in a predetermined range as a center of the working point due to the movement of the gear 11, the resistance changes correspondingly, too. For example, the magnetoresistive effect element (portion) MRE1 on the nearest side to the gear 11, the magnetoresistive effect element (portion) MREA at a point A, and the magnetoresistive effect element (portion) MREn on the farthest side to the gear 11 have respective working points as shown in FIG. 7.
As to the changes of the resistance value with respect to changes of the magnetic field intensity of an identical range, the portion MRE1 represents the smallest value, the portion MREa an intermediate value, and the portion MREn the biggest value, as shown in FIG. 8. The resistance change of the magnetoresistive effect element 16a exhibits a waveform shown in FIG. 9, because of the composition of the portions MRE1 to MREn. Thus, since the resultant waveform has distortions as shown in the figure, there is a problem that the stable sensor output cannot be realized.